Foreign object detection in a wireless power transfer system

ABSTRACT

A power transmitter (101) for a wireless power transfer system comprises a transmitter coil (103) and a driver (201) generates a drive signal for the transmitter coil (103) employing a repeating time frame with a power transfer time interval and a foreign object detection time interval. A test generator (211) generates a test drive signal for a test coil (209) during the foreign object detection time interval. A foreign object detector (207) performs a foreign object detection test based on a measured parameter for the test drive signal. Prior to entering a power transfer phase, an adapter (213) controls the power transmitter (101) to operate in a foreign object detection initialization mode in which a preferred value of a signal parameter for the test drive signal is determined in response to at least a first message received from the power receiver (105). During the foreign object detection time interval the signal parameter of is set to the preferred value.

FIELD OF THE INVENTION

The invention relates to foreign object detection in a wireless powertransfer system and in particular, but not exclusively, to foreignobject detection for a power transmitter providing inductive powertransfer to higher power devices, such as e.g. kitchen appliances.

BACKGROUND OF THE INVENTION

Most present-day electrical products require a dedicated electricalcontact in order to be powered from an external power supply. However,this tends to be impractical and requires the user to physically insertconnectors or otherwise establish a physical electrical contact.Typically, power requirements also differ significantly, and currentlymost devices are provided with their own dedicated power supplyresulting in a typical user having a large number of different powersupplies with each power supply being dedicated to a specific device.Although, the use of internal batteries may avoid the need for a wiredconnection to a power supply during use, this only provides a partialsolution as the batteries will need recharging (or replacing). The useof batteries may also add substantially to the weight and potentiallycost and size of the devices.

In order to provide a significantly improved user experience, it hasbeen proposed to use a wireless power supply wherein power isinductively transferred from a transmitter inductor in a powertransmitter device to a receiver coil in the individual devices.

Power transmission via magnetic induction is a well-known concept,mostly applied in transformers having a tight coupling between a primarytransmitter inductor/coil and a secondary receiver coil. By separatingthe primary transmitter coil and the secondary receiver coil between twodevices, wireless power transfer between these becomes possible based onthe principle of a loosely coupled transformer.

Such an arrangement allows a wireless power transfer to the devicewithout requiring any wires or physical electrical connections to bemade. Indeed, it may simply allow a device to be placed adjacent to, oron top of, the transmitter coil in order to be recharged or poweredexternally. For example, power transmitter devices may be arranged witha horizontal surface on which a device can simply be placed in order tobe powered.

Furthermore, such wireless power transfer arrangements mayadvantageously be designed such that the power transmitter device can beused with a range of power receiver devices. In particular, a wirelesspower transfer approach, known as the Qi Specifications, has beendefined and is currently being developed further. This approach allowspower transmitter devices that meet the Qi Specifications to be usedwith power receiver devices that also meet the Qi Specifications withoutthese having to be from the same manufacturer or having to be dedicatedto each other. The Qi standard further includes some functionality forallowing the operation to be adapted to the specific power receiverdevice (e.g. dependent on the specific power drain).

The Qi Specification is developed by the Wireless Power Consortium andmore information can e.g. be found on their website:http://www.wirelesspowerconsortium.com/index.html, where in particularthe defined Specification documents can be found.

A potential problem with wireless power transfer is that power mayunintentionally be transferred to e.g. metallic objects that happen tobe in the vicinity of the power transmitter. For example, if a foreignobject, such as e.g. a coin, key, ring etc., is placed upon the powertransmitter platform arranged to receive a power receiver, the magneticflux generated by the transmitter coil will introduce eddy currents inthe metal objects which will cause the objects to heat up. The heatincrease may be very significant and may be highly disadvantageous.

In order to reduce the risk of such scenarios arising, it has beenproposed to introduce foreign object detection where the powertransmitter can detect the presence of a foreign object and reduce thetransmit power and/or generate a user alert when a positive detectionoccurs. For example, the Qi system includes functionality for detectinga foreign object, and for reducing power if a foreign object isdetected. Specifically, Qi specification version 1.2.1, section 11describes various methods of detecting a foreign object.

One method to detect such foreign objects is disclosed inWO2015018868A1. Another example is provided in WO 2012127335 whichdiscloses an approach based on determining unknown power losses. In theapproach, both the power receiver and the power transmitter measuretheir power, and the receiver communicates its measured received powerto the power transmitter. When the power transmitter detects asignificant difference between the power sent by the transmitter and thepower received by the receiver, an unwanted foreign object maypotentially be present, and the power transfer may be reduced or abortedfor safety reasons. This power loss method requires synchronizedaccurate power measurements performed by the power transmitter and thepower receiver.

For example, in the Qi power transfer standard, the power receiverestimates its received power e.g. by measuring the rectified voltage andcurrent, multiplying them and adding an estimate of the internal powerlosses in the power receiver (e.g. losses of the rectifier, the receivercoil, metal parts being part of the receiver etc.). The power receiverreports the determined received power to the power transmitter with aminimum rate of e.g. every four seconds.

The power transmitter estimates its transmitted power, e.g. by measuringthe DC input voltage and current of the inverter, multiplying them andcorrecting the result by subtracting an estimation of the internal powerlosses in the transmitter, such as e.g. the estimated power loss in theinverter, the primary coil, and metal parts that are part of the powertransmitter.

The power transmitter can estimate the power loss by subtracting thereported received power from the transmitted power. If the differenceexceeds a threshold, the transmitter will assume that too much power isdissipated in a foreign object, and it can then proceed to terminate thepower transfer.

Alternatively, it has been proposed to measure the quality or Q-factorof the resonant circuit formed by the primary and secondary coilstogether with the corresponding capacitances and resistances. Areduction in the measured Q-factor may be indicative of a foreign objectbeing present.

In practice, it tends to be difficult to achieve sufficient detectionaccuracy using the methods described in the Qi specification. Thisdifficulty is exacerbated by a number of uncertainties about thespecific current operating conditions.

For example, a particular problem is the potential presence of friendlymetals (i.e. metal parts of the device that embodies the power receiveror the power transmitter) as the magnetic and electrical properties ofthese may be unknown (and vary between different devices) and thereforemay be difficult to compensate for.

Further, undesirable heating may result from even relatively smallamounts of power being dissipated in a metallic foreign object.Therefore, it is necessary to detect even a small power discrepancybetween the transmitted and received power, and this may be particularlydifficult when the power levels of the power transfer increase.

The Q factor degradation approach may in many scenarios have a bettersensitivity for detecting the presence of metal objects. However, it maystill not provide sufficient accuracy and e.g. may also suffer from theinfluence of friendly metal.

The performance of the foreign object detection is subject to thespecific operating conditions that are present when the test is actuallyperformed. For example, if, as described in the Qi specification, ameasurement for foreign object detection is performed in the SelectionPhase of a power transfer initialization process, the signal that thepower transmitter provides for the measurement must be small enough toprevent that it wakes up the power receiver. However, for such a smallsignal, the signal/noise ratio is typically poor, resulting in reducedaccuracy of the measurement.

The requirement for a small measurement signal may result in otherdisadvantageous effects. A power receiver exposed to a small measurementsignal may exhibit a leakage current that depends on the level of themeasurement signal, the coupling between the primary and secondary coil,and the charging state of a capacitor at the output of the rectifier.This leakage current can therefore be different depending on the actualconditions. Since leakage current influences the reflected impedance atthe power transmitter coil, the measurement of the quality factor willalso depend on the specific current conditions.

Another issue is that foreign object detection is typically a verysensitive test where it is desired that relatively small changes causedby the presence of a foreign object is detected in an environment withpossibly a large variation of the operating conditions and scenarios forwhich the test is being performed.

Accordingly, current algorithms tend to be suboptimal and may in somescenarios and examples provide less than optimum performance. Inparticular, they may result in the presence of foreign objects not beingdetected, or in false detections of foreign objects when none arepresent.

Hence, an improved object detection would be advantageous and, inparticular, an approach allowing increased flexibility, reduced cost,reduced complexity, improved object detection, fewer false detectionsand missed detections, backwards compatibility, and/or improvedperformance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination.

According to an aspect of the invention there is provided powertransmitter for wirelessly providing power to a power receiver via aninductive power transfer signal; the power transmitter comprising: atransmitter coil for generating the power transfer signal; a driver forgenerating a drive signal for the transmitter coil, the driver beingarranged to, during a power transfer phase, generate the drive signal toemploy a repeating time frame comprising at least a power transfer timeinterval and a foreign object detection time interval; a receiver forreceiving messages from the power receiver; a test coil for generatingan electromagnetic test signal; a test generator arranged to generate atest drive signal for the test coil to provide the electromagnetic testsignal during the foreign object detection time interval; a foreignobject detector arranged to perform a foreign object detection test inresponse to a measured parameter for the test drive signal; an adapterfor, prior to entering the power transfer phase, control the powertransmitter to operate in a foreign object detection initialization modein which a preferred value of a signal parameter for the test drivesignal is determined in response to at least a first message receivedfrom the power receiver; and wherein the test generator is arranged toset the signal parameter of the test drive signal to the preferred valueduring the foreign object detection time interval.

The invention may provide improved foreign object detection in manyembodiments. In many scenarios and systems, a more accurate foreignobject detection may be achieved. The approach may in many embodimentsreduce complexity and may in many systems provide a high degree ofbackwards compatibility. Specifically, the approach may be particularlysuitable for improving foreign object detection in Qi wireless powertransfer systems e.g. operating in accordance with version 1.2 orearlier of the Qi Specifications.

The approach may allow for improved accuracy and/or reliability offoreign object detection tests during the power transfer phase. In manyembodiments, the approach may reduce uncertainty and variation for theforeign object detection tests thereby improving performance. Theapproach mays specifically reduce the impact on power transfervariations and operating conditions on the foreign object detection. Theapproach may for example bias the system towards working at a specific,e.g. predetermined, reference scenario and operating point during theforeign object detection. This may improve consistency andpredictability for the foreign object detection test. In particular, itmay allow a more accurate and more reliable estimation of the impact ofthe power receiver on the electromagnetic test signal, and thus mayallow the foreign object detector to improve compensation therefor.

In many embodiments, the test coil and the transmitter coil may be thesame coil. In many embodiments, the driver and the test generator may bethe same entity, thus the same circuitry may generate both the drivesignal and the test drive signal. In many embodiments, the powertransfer signal and the test drive signal may share many parametervalues, for example they may have the same frequency.

The signal parameter for which a preferred value is determined mayspecifically be a frequency, voltage, current, signal level, phase,timing, and/or amplitude.

The preferred value may be any value that is determined by the adapterfor the signal parameter, and may equivalently be referred to as e.g.the first parameter.

In many embodiments, a duration of the foreign object detection timeinterval is no more than 5%, 10%, or 20% of the duration of the timeframe. In many embodiments, the duration of the foreign object detectiontime interval(s) is no less than 70%, 80%, or 90% of the time frame. Theadapter may control the power transmitter to operate in the foreignobject detection initialization mode in an adaptation time interval. Inmany embodiments, the duration of the foreign object detection timeinterval is no more than 5%, 10%, or 20% of the duration of theadaptation time interval.

The approach may e.g. introduce a foreign object detection time intervalin which the power receiver can operate with both a high induced voltageyet light load, corresponding to a high magnetic field strength yet lowloading of the electromagnetic signal. In such scenarios, the impact ofa foreign object may be more noticeable as power induced in such anobject will represent a higher proportion of the total power extracted.Indeed, the higher magnetic strength may result in a higher inducedsignal in any foreign object being present, and the reduced loading mayreduce the impact of the presence of the power receiver when detectingwhether a foreign object is present.

The foreign object detector may be arranged to determine that a foreignobject is detected if a difference between the power level of theelectromagnetic test signal and the power indicated by a loadingindication received from the power receiver and indicating an expectedload of the electromagnetic test signal is above a threshold. If thedifference is below the threshold, the foreign object detector maydetermine that no foreign object is detected.

The foreign object detector may be arranged to determine that a foreignobject is detected if a quality measure (determined from measurements ofthe drive signal) for a resonance circuit comprising the test coil isbelow a threshold. The threshold may typically be dependent on a messagereceived from the power receiver.

In accordance with an optional feature of the invention, the firstmessage comprises an indication of a property of the power receiver.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test. It may in particular allow the foreign object detectiontest to provide improved compensation for the specific properties of thepower receiver when determining the preferred value. It may also allowthe foreign object detection test to compensate for properties of theindividual power receiver.

The property may for example be an indication of a type/class/categoryof power receiver and/or may be a power receiver identification. In someembodiments, the indication of the property may be indicative of anestimated loading of the electromagnetic test signal by the powerreceiver, such as e.g. an indication of the loading by friendly metal ofthe power receiver. In some embodiments, the indication of the propertymay be an indication of a preferred parameter setting, such as e.g. anindication of a resonance frequency of a resonance circuit of the powerreceiver including a power receiving coil.

In some embodiments, the indication of the property may be indicative ofan estimated loading of the electromagnetic test signal by the powerreceiver, such as e.g. an indication of the loading by friendly metal ofthe power receiver.

In some embodiments, the indication of the property may be indicative ofan estimated loading of the electromagnetic test signal by the powerreceiver, such as e.g. an indication of the loading by friendly metal ofthe power receiver. In some embodiments, the indication of the propertymay be indicative of an estimated impact on a quality measure for aresonance circuit comprising the transmitter coil, such as e.g. anindication of the impact of friendly metal of the power receiver.

In accordance with an optional feature of the invention, the firstmessage comprises an indication of an expected impact of the powerreceiver on a reference electromagnetic test signal.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test. It may in particular allow the foreign object detectiontest to provide improved compensation for the impact of the powerreceiver when determining the preferred value. It may also allow theforeign object detection test to compensate for the impact of theindividual power receiver.

In some embodiments, the indication of the property may be indicative ofan estimated loading of the electromagnetic test signal by the powerreceiver, such as e.g. an indication of the loading by friendly metal ofthe power receiver.

In some embodiments, the indication of the property may be indicative ofan estimated loading of the electromagnetic test signal by the powerreceiver, such as e.g. an indication of the loading by friendly metal ofthe power receiver. In some embodiments, the indication of the propertymay be indicative of an estimated impact on a quality measure for aresonance circuit comprising the transmitter coil, such as e.g. anindication of the impact of friendly metal of the power receiver.

The indication of the expected impact may for example be an indicationof an expected power dissipation of the power receiver during theforeign object detection time interval, or an indication of an expectedimpact on a quality measure for a resonance circuit comprising the testcoil.

In accordance with an optional feature of the invention, the firstmessage comprises an indication of a constraint for the signal parameterfor the test drive signal.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test. It may in particular allow the test drive signal andthus the electromagnetic test signal to be generated to have propertiesthat allow improved foreign object detection and/or may ensureacceptable operation (e.g. sufficient power during foreign objectdetection).

The constraint may specifically be a constraint on a signal level(current, voltage and/or power) or frequency (e.g. maximum and/orminimum) of the test drive signal.

In accordance with an optional feature of the invention, the firstmessage comprises an indication of a difference between a current powerreceiver operating value and a test reference power receiver operatingvalue.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test. It may in particular allow the power transmitter togenerate the test drive signal to result in properties of theelectromagnetic test signal corresponding to a desired reference level(or interval).

In accordance with an optional feature of the invention, the adapter isfurther arranged to determine the preferred value in response toconstraint of the foreign object detection test of the foreign objectdetector.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test. In particular, it may ensure that the foreign objectdetection is performed with a suitable electromagnetic test signal forthe test even if the power receiver requests or indicates that othervalues should be used.

In accordance with an optional feature of the invention, the constraintis at least one of a minimum signal level and a constraint on afrequency of the test drive signal.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test.

In accordance with an optional feature of the invention, the testgenerator is arranged to generate the test drive signal with the driveparameter of the test drive signal adapted to the preferred value in aninitial test interval prior to the power transfer phase; and the foreignobject detector is arranged to perform the foreign object detection testin the initial test interval.

This may provide improved operation and may specifically typically avoidinitialization of the power transfer phase with a foreign objectpresent. It may also increase reliability and reduce the risk of thesignal parameter being set to a value that is not suitable for foreignobject detection.

In accordance with an optional feature of the invention, if the foreignobject detection test in the foreign object detection time interval isindicative of a foreign object being present, the foreign objectdetector is arranged to re-enter the power transmitter into the foreignobject detection initialization mode.

This may provide improved operation in many scenarios. For example, itmay in many embodiments allow the system to automatically re-calibrateto sudden changes in the operating conditions, such as e.g. caused by amovement of the power receiver relatively to the power transmitter.

In accordance with an optional feature of the invention, the adapter isarranged to prevent the power transmitter from entering the powertransfer phase if the preferred value does not meet a criterion.

This may provide improved operation and may specifically typically avoidinitialization of the power transfer phase with a foreign objectpresent. It may also increase reliability and reduce the risk of thesignal parameter being set to a value that is not suitable for foreignobject detection (or potentially any other operation).

In accordance with an optional feature of the invention, the foreignobject detector is arranged to adapt a parameter of the foreign objectdetection test in response to a measured value of the drive signal whenin the foreign object detection initialization mode.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test.

In accordance with an optional feature of the invention, the adapter isarranged to set a maximum level for the power transfer signal during thepower transfer interval in response to a measurement of the test drivesignal during the foreign object detection interval.

This may provide improved operation in many embodiments.

According to an aspect of the invention there is provided a wirelesspower transfer system comprising a power transmitter for wirelesslyproviding power to a power receiver via an inductive power transfersignal; the power transmitter comprising: a transmitter coil forgenerating the power transfer signal; a driver for generating a drivesignal for the transmitter coil, the driver being arranged to, during apower transfer phase, generate the drive signal to employ a repeatingtime frame comprising at least a power transfer time interval and aforeign object detection time interval, a power of the power transfersignal being reduced during the foreign object detection time intervalrelative to the power transfer time interval; a receiver for receivingmessages from the power receiver; a test coil for generating anelectromagnetic test signal; a test generator arranged to generate atest drive signal for the test coil to provide the electromagnetic testsignal during the foreign object detection time interval; a foreignobject detector arranged to perform a foreign object detection test inresponse to a measured parameter for the test drive signal; an adapterfor, prior to entering the power transfer phase, control the powertransmitter to operate in a foreign object detection initialization modein which a preferred value of a signal parameter for the test drivesignal is determined in response to at least a first message receivedfrom the power receiver; and wherein the test generator is arranged toset the signal parameter of the test drive signal to the preferred valueduring the foreign object detection time interval; and the powerreceiver comprising: a power receiving coil for extracting power fromthe power transfer signal; a foreign object detection controller forreducing a load of the power receiver during the foreign objectdetection time interval; a message transmitter for transmitting thefirst message to the power transmitter.

In accordance with an optional feature of the invention, the powerreceiver further comprises a power receiver controller arranged tocontrol the power receiver to operate in a foreign object detectioninitialization mode in which the power receiver transmits at least onemessage to the power transmitter to bias the test drive signal towardscausing a reference condition at the power receiver.

This may provide improved foreign object detection, and in particularmay provide improved reliability and accuracy of the foreign objectdetection test.

In many embodiments, the adapter may be arranged to determine thepreferred value of the signal parameter in response to a measurement ofthe test drive signal for the power receiver being at the referencecondition.

According to an aspect of the invention there is provided a method ofoperation for a power transmitter wirelessly providing power to a powerreceiver via an inductive power transfer signal; the power transmittercomprising: a transmitter coil for generating the power transfer signal,a test coil for generating an electromagnetic test signal, and areceiver for receiving messages from the power receiver; and the methodcomprising: generating a drive signal for the transmitter coil, thedrive signal, during a power transfer phase, employing a repeating timeframe comprising at least a power transfer time interval and a foreignobject detection time interval; generating a test drive signal for thetest coil to provide the electromagnetic test signal during the foreignobject detection time interval; performing a foreign object detectiontest in response to a measured parameter for the test drive signal; andprior to entering the power transfer phase, controlling the powertransmitter to operate in a foreign object detection initialization modein which a preferred value of a signal parameter for the test drivesignal is determined in response to at least a first message receivedfrom the power receiver; and wherein the test drive signal is generatedwith the signal parameter set to the preferred value during the foreignobject detection time interval

These and other aspects, features and advantages of the invention willbe apparent from and elucidated with reference to the embodiment(s)described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 illustrates an example of elements of a power transfer system inaccordance with some embodiments of the invention;

FIG. 2 illustrates an example of elements of a power transmitter inaccordance with some embodiments of the invention;

FIG. 3 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention;

FIG. 4 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention;

FIG. 5 illustrates an example of a time frame for a wireless powertransfer system of FIG. 1; and

FIG. 6 illustrates an example of a time frame for a wireless powertransfer system of FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description focuses on embodiments of the inventionapplicable to a wireless power transfer system utilizing a powertransfer approach such as known from the Qi specification. However, itwill be appreciated that the invention is not limited to thisapplication but may be applied to many other wireless power transfersystems.

FIG. 1 illustrates an example of a power transfer system in accordancewith some embodiments of the invention. The power transfer systemcomprises a power transmitter 101 which includes (or is coupled to) atransmitter coil/inductor 103. The system further comprises a powerreceiver 105 which includes (or is coupled to) a receiver coil/inductor107.

The system provides an electromagnetic power transfer signal which mayinductively transfer power from the power transmitter 101 to the powerreceiver 105. Specifically, the power transmitter 101 generates anelectromagnetic signal, which is propagated as a magnetic flux by thetransmitter coil or inductor 103. The power transfer signal maytypically have a frequency between around 20 kHz to around 500 kHz, andoften for Qi compatible systems typically in the range from 95 kHz to205 kHz (or e.g. for high power kitchen applications, the frequency maye.g. typically be in the range between 20 kHz to 80 kHz). Thetransmitter coil 103 and the power receiving coil 107 are looselycoupled and thus the power receiving coil 107 picks up (at least partof) the power transfer signal from the power transmitter 101. Thus, thepower is transferred from the power transmitter 101 to the powerreceiver 105 via a wireless inductive coupling from the transmitter coil103 to the power receiving coil 107. The term power transfer signal ismainly used to refer to the inductive signal/magnetic field between thetransmitter coil 103 and the power receiving coil 107 (the magnetic fluxsignal), but it will be appreciated that by equivalence it may also beconsidered and used as a reference to an electrical signal provided tothe transmitter coil 103 or picked up by the power receiving coil 107.

In the example, the power receiver 105 is specifically a power receiverthat receives power via the receiver coil 107. However, in otherembodiments, the power receiver 105 may comprise a metallic element,such as a metallic heating element, in which case the power transfersignal directly induces eddy currents resulting in a direct heating ofthe element.

The system is arranged to transfer substantial power levels, andspecifically the power transmitter may support power levels in excess of500 mW, 1 W, 5 W, 50 W, 100 W or 500 W in many embodiments. For example,for Qi corresponding applications, the power transfers may typically bein the 1-5 W power range for low power applications (the basic powerprofile), up to 15 W for Qi specification version 1.2, in the range upto 100 W for higher power applications such as power tools, laptops,drones, robots etc., and in excess of 100 W and up to more than 1000 Wfor very high power applications, such as e.g. kitchen applications.

In the following, the operation of the power transmitter 101 and thepower receiver 105 will be described with specific reference to anembodiment generally in accordance with the Qi Specification (except forthe herein described (or consequential) modifications and enhancements)or suitable for the higher power kitchen specification being developedby the Wireless Power Consortium. In particular, the power transmitter101 and the power receiver 105 may follow, or substantially becompatible with, elements of the Qi Specification version 1.0, 1.1 or1.2 (except for the herein described (or consequential) modificationsand enhancements).

In wireless power transfer systems, the presence of an object (typicallya conductive element extracting power from the power transfer signal andnot being part of the power transmitter 101 or the power receiver 105,i.e. being an unintended, undesired, and/or interfering element to thepower transfer) may be highly disadvantageous during a power transfer.Such an undesired object is in the field known as a foreign object.

A foreign object may not only reduce efficiency by adding a power lossto the operation but may also degrade the power transfer operationitself (e.g. by interfering with the power transfer efficiency orextracting power not directly controlled e.g. by the power transferloop). In addition, the induction of currents in the foreign object(specifically eddy currents in the metal part of a foreign object) mayresult in an often highly undesirable heating of the foreign object.

In order to address such scenarios, wireless power transfer systems suchas Qi include functionality for foreign object detection. Specifically,the power transmitter comprises functionality seeking to detect whethera foreign object is present. If so, the power transmitter may e.g.terminate the power transfer or reduce the maximum amount of power thatcan be transferred.

Current approaches proposed by the Qi Specifications are based ondetecting a power loss (by comparing the transmitted and the reportedreceived power) or detecting degradations in the quality Q of the outputresonance circuit. However, in current use these approaches have beenfound to provide suboptimal performance in many scenarios, and they mayspecifically lead to inaccurate detection resulting in missed detectionsand/or false positives where a foreign object is detected despite nosuch object being present.

Foreign object detection may be performed before a power receiver entersthe power transfer phase (e.g. during the initialization of the powertransfer) or during the power transfer phase. Detection during the powertransfer phase is often based on comparisons of measured transmittedpower and received power whereas detection that take place before thepower transfer phase is often based on measurements of a reflectedimpedance, e.g. by measuring the quality factor of the transmitter coilby using a small measurement signal.

The inventors have realized that conventional foreign object detectionoperates suboptimally and that this is partly due to variations anduncertainties in the specific operating conditions and scenario in whichthe foreign object detection is performed, including variations anduncertainties in the power transmitter properties, power receiverproperties, test conditions applied etc.

An example of the challenges to foreign object detection tests is therequirement to perform sufficiently accurate measurements in order toachieve a sufficiently reliable foreign object detection. For example,if a measurement for a foreign object detection takes place in theselection phase of a Qi power transfer initialization phase, the signalthat the power transmitter provides for this measurement has to be smallenough not to wake up the power receiver. However, this typically resultin poor signal/noise ratios leading to reduced detection accuracy.Therefore, the detection performance may be sensitive to the specificsignal level applied and there will typically be conflictingrequirements.

A power receiver exposed to a small electromagnetic signal may show aleakage current that depends on the level of the electromagnetic signal,the coupling between the primary and secondary coil, and the chargingstate of the capacitor at the output of the rectifier. This leakagecurrent can therefore vary depending on the actual conditions currentlyexperienced and depending on the specific parameters (e.g. properties ofcapacitor) of the individual power receiver. Since leakage currentinfluences the reflected impedance at the primary coil, the measurementof the quality factor also depends on the actual conditions and thistypically prevents optimal detection.

Yet another problem detecting a foreign object based on e.g. reportedreceived power indications at different loads or signal levels can beless reliable than desired due to the relationships between transmittedand received power being different for different loads and signalslevels.

The system of FIG. 1 uses an approach for foreign object detection thatseeks to reduce uncertainty and sensitivity to variations, andaccordingly it seeks to provide improved foreign object detection. Theapproach may in many embodiments provide improved foreign objectdetection and specifically may in many embodiments provide a moreaccurate and/or reliable foreign object detection. The approach mayfurther allow low complexity and low resource requirements. An advantageof the approach is that it may be suitable for inclusion in manyexisting systems, such as specifically in a Qi wireless power transfersystem, and indeed that this may often be achieved with fewmodifications.

As will be described in more detail in the following, the approachutilizes a time division approach during the power transfer phasewherein foreign object detection and power transfer may e.g. beperformed in separate time intervals thereby allowing the interferencebetween these (specifically the impact of the power transfer on theforeign object detection) to be reduced substantially. Furthermore,parameters of the generated electromagnetic signal may be adapted to thespecific test scenario, including parameters of potentially both thepower transmitter and the power receiver. This can be achieved throughan adaptation process which is performed prior to the system enteringthe power transfer phase wherein one or more preferred parameter valuesof the test signal are determined based on at least one message receivedfrom the power receiver.

The approach may substantially reduce variations and uncertainties andlead to typically a much more accurate foreign objection detection.

For example, the influence and corresponding uncertainty of the powerreceiver's load can be reduced or even eliminated by the power receiverdisconnecting the load during foreign object detection time intervalswhen the foreign object detection is performed. Although this may e.g.result in discontinuous power provisions, this may be overcome by anenergy buffer, such as a large capacitor, providing power duringtypically short foreign object detection intervals.

As another example, e.g. the frequency or signal level of the testsignal generated during the foreign object detection time interval mayduring the pre-power transfer phase be determined such that it at thereceiver will correspond to a specific reference operatingpoint/condition. The power receiver parameters/properties may be wellknown for this specific setting and accordingly these can be compensatedfor/considered in the foreign object detection test resulting in a morereliable and accurate test.

Yet another problem of existing methods is the difference between powertransmitter designs and a reference power transmitter design for whichthe foreign object detection test has been designed or based on whichthe test parameters have been determined (e.g. via the technicalspecifications of the system). This may e.g. lead to differences in ameasured quality factor at the power transmitter compared to thereference power transmitter. The power transmitter accordingly cannotdirectly use the information on an expected quality factor/reference Qfactor received from the power receiver. Indeed, the power transmitterneeds to translate the measured Q factor to the corresponding Q factorof the reference power transmitter, or translate the received referencequality factor to a new value that makes sense for its measured Qfactor. Moreover, although the Q factor gives an indication of the powerloss in a foreign object as seen from the power coil of the transmitter,it also depends on other aspects that are not directly related to theheating of a foreign object. However, by appropriately determiningparameters for the measurement signal during the pre-power transferphase adaptation, the measurement signal may be set for potentiallycompensating for such differences.

In the following, the system of FIG. 1 will be described in more detail.In the example, the electromagnetic power transfer signal and theelectromagnetic test signal used for the foreign object detection aregenerated by two different coils (driven by different drivers). Further,the signals will be referred to by different terms, namely theelectromagnetic signal generated during power transfer time intervalswill be referred to as the power transfer signal and the electromagneticsignal generated during foreign object detection time intervals will bereferred to as the electromagnetic test signal, or just the test signal.However, it will be appreciated that in many embodiments, theelectromagnetic signal may be generated from the same coil in both thepower transfer time interval and the foreign object detection timeinterval, and indeed the same driver etc. may be used for both the powertransfer time interval and the foreign object detection time interval.Indeed, the references to the test signals may in many embodiments beconsidered equivalent to the power transfer signal during the foreignobject detection time interval.

FIG. 2 illustrates elements of the power transmitter 101 and FIG. 3illustrates elements of the power receiver 105 of FIG. 1 in more detail.

The power transmitter 101 includes a driver 201 which can generate adrive signal that is fed to the transmitter coil 103 which in returngenerates the electromagnetic power transfer signal, which can provide apower transfer to the power receiver 105. The power transfer signal isprovided during power transfer time intervals of the power transferphase.

The driver 201 may typically comprise an output circuit in the form ofan inverter, typically formed by driving a full or half bridge as willbe well known to the skilled person.

The power transmitter 101 further comprises a power transmittercontroller 203 which is arranged to control the operation of the powertransmitter 101 in accordance with the desired operating principles.Specifically, the power transmitter 101 may include many of thefunctionalities required to perform power control in accordance with theQi Specifications.

The power transmitter controller 203 is in particular arranged tocontrol the generation of the drive signal by the driver 201, and it canspecifically control the power level of the drive signal, andaccordingly the level of the generated power transfer signal. The powertransmitter controller 203 comprises a power loop controller controllinga power level of the power transfer signal in response to the powercontrol messages received from the power receiver 105 during the powercontrol phase.

In order to receive data and messages from the power receiver 105, thepower transmitter 101 comprises a message receiver 205 which is arrangedto receive data and messages from the power receiver 105 (as will beappreciated by the skilled person, a data message may provide one ormore bits of information). In the example, the power receiver 105 isarranged to load modulate the power transfer signal generated by thetransmitter coil 103, and the message receiver 205 is arranged to sensevariations in the voltage and/or current of the transmitter coil 103 andto demodulate the load modulation based on these. The skilled personwill be aware of the principles of load modulation, as e.g. used in Qiwireless power transfer systems, and therefore these will not bedescribed in further detail.

In some embodiments, communication may be performed using a separatecommunication channel which may be achieved using a separatecommunication coil, or indeed using the transmitter coil 103. Forexample, in some embodiments Near Field Communication may be implementedor a high frequency carrier (e.g. with a carrier frequency of 13.56 MHz)may be overlaid on the power transfer signal.

The power transmitter 101 further comprises a foreign object detector207 which is arranged to perform foreign object detection tests, i.e. tospecifically detect whether any undesired conductive elements are likelyto be present within the generated electromagnetic field.

In the system, the foreign object detection tests are based onmeasurements performed during foreign object detection time intervals.During these foreign object detection time intervals, the transmittercontroller 203 is arranged to reduce the power level of the powertransfer signal, and specifically it may in the example of usingdifferent coils for generating the power transfer signal and theelectromagnetic test signal switch the power transfer signal offcompletely.

During an interval in which foreign object detection is performed, i.e.during a foreign object detection time interval, the foreign objectdetector 207 may evaluate conditions to determine whether a foreignobject is considered present or not. During the foreign object detectiontime interval, the power transmitter 101 generates an electromagnetictest signal and the foreign object detection is based on evaluatingcharacteristics and properties of this signal.

For example, the power level of (the power extracted from) the generatedtest signal may be used as an indication of the power being extracted bypotential foreign objects (typically by comparing it to the expectedpower extraction from the power receiver 105). The power level of theelectromagnetic test signal reflects the power that is extracted fromthe electromagnetic test signal by conductive elements (including thereceiver coil 107) in the electromagnetic field. It thus indicates thepower extracted by the combination of the power receiver 105 as well asany foreign objects that may be present. The difference between thepower level of the electromagnetic signal and the power extracted by thepower receiver 105 accordingly reflects the power extracted by anyforeign objects present. The foreign object detection may for example bea low complexity detection wherein a detection of a foreign object isconsidered to have occurred if the difference between the power level ofthe electromagnetic signal (henceforth referred to as transmit powerlevel) exceeds the reported power extracted by the power receiver 105(henceforth referred to as received power level).

In the approach, the foreign object detection is accordingly based on apower level comparison between a transmitted power level and a reportedreceived power level. The reaction to a detection of a foreign objectmay be different in different embodiments. However, in many embodiments,the power transmitter 101 may be arranged to terminate a power transfer(at least temporarily) in response to a detection of a foreign object.

In order to generate the test signal, the power transmitter 101comprises a test coil 209 which is coupled to a test generator 211. Thetest generator 211 is arranged to generate a test drive signal for thetest coil 209 to provide the electromagnetic test signal during theforeign object detection time interval. The test drive signal is anelectrical signal fed to the test coil 209 resulting in theelectromagnetic test signal being generated, i.e. the test coil 209generates a corresponding electromagnetic field with a field strengthdepending on the test drive signal.

The test generator 211 may be comprise substantially the samefunctionality as the driver 201, e.g. the output of the test generator211 may be a half or full bridge inverter. Indeed, as previouslymentioned, in many embodiments, the test generator 211 may beimplemented by the driver 201 and the test coil 209 may be implementedby the transmitter coil 103. Accordingly, in the following, allreferences to test generator 211 and the test coil 209 may asappropriate be considered as references to the driver 201 and the testcoil 209 for embodiments where the same coil is used for the generationof both the power transfer signal and the electromagnetic test signal.In such a situation, the power of the generated electromagnetic signalmay be adapted to typically a fixed reference level during the foreignobject detection time interval relative to the power transfer timeinterval.

The power transmitter further comprises an adapter which is arranged to,prior to the power transmitter 101 entering the power transfer phase,determine a suitable value for one or more parameters of the test drivesignal. These values are then applied during (at least one) foreignobject detection time intervals of the power transfer phase. The adapter213 will be described in more detail later.

FIG. 3 illustrates some exemplary elements of the power receiver 105.

The receiver coil 107 is coupled to a power receiver controller 301which couples the receiver coil 107 to a load 303 via a switch 305 (i.e.it is a switchable load 305). The power receiver controller 301 includesa power control path which converts the power extracted by the receivercoil 107 into a suitable supply for the load. In addition, the powerreceiver controller 301 may include various power receiver controllerfunctionality required to perform power transfer, and in particularfunctions required to perform power transfer in accordance with the Qispecifications.

In order to support communication from the power receiver 105 to thepower transmitter 101 the power receiver 105 comprises a load modulator307. The load modulator 307 is arranged to vary the loading of thereceiver coil 107 in response to data to be transmitted to the powertransmitter 101. The load variations are then detected and demodulatedby the power transmitter 101 as will be known to the person skilled inthe art.

FIG. 4 illustrates a circuit diagram of elements of an example of apower path of the power receiver 105. In the example, the power receiver105 comprises the receiver coil 107 referred to by the designation LRX.In the example, receiver coil 107 is part of a resonance circuit and thepower receiver 105 accordingly also includes a resonance capacitor CRX.The receiver coil 107 is subjected to the electromagnetic signal andaccordingly an AC voltage/current is induced in the coil. The resonancecircuit is coupled to a rectifier bridge with a smoothing capacitor C1coupled to the output of the bridge. Thus, a DC voltage is generatedover the capacitor C1. The magnitude of the ripple on the DC voltagewill depend on the size of the smoothing capacitor as well as on theload.

The bridge B1 and smoothing capacitor C1 are coupled to the load 303which is indicated by reference sign RL via the switch 305 which isillustrated by switch S1. The switch 305 can accordingly be used toconnect or disconnect the load from the power path and thus the load isa switchable load 305. It will be appreciated that whereas the switch S1is shown as a conventional switch, it may of course be implemented byany suitable means including typically by a MOSFET. It will also beappreciated that the load 303 is illustrated as a simple passive portbut that it may of course be any suitable load. For example, the load303 may be a battery to be charged, a mobile phone, or anothercommunication or computational device, may be a simple passive load etc.Indeed, the load 303 need not be an external or dedicated internal loadbut may for example include elements of the power receiver 105 itself.Thus, the load 303 illustrated in FIGS. 3 and 4 may be considered torepresent any load of the receiver coil 107/the electromagnetic signalthat can be disconnected by the switch 305/S1, and it is accordinglyalso referred to as a switchable load 305.

FIG. 4 further illustrates a load modulation capacitor C2 which can beconnected or disconnected in parallel to the resonance circuit based onthe switching of switch S2. The load modulator 307 may be arranged tocontrol the switch S2 such that the load of the modulation capacitor C2can be connected and disconnected in response to data to be transmittedto the power transmitter 101 thereby providing load modulation.

The power receiver 105 is arranged to enter a foreign object detectionmode during the foreign object detection time interval of each timeframe during the power transfer phase. In the example, the powerreceiver 105 comprises a load controller 309 which controls the switch305 (equivalently the switch 305 can be considered part of the loadcontroller). During a foreign object detection time interval, the loadcontroller 309 can disconnect the load 303 from the power receiver, i.e.it disconnects a load of the power receiver controller 301, and thus aload of the receiver coil 107. Thus, in this way the load controller 309may reduce the loading of the receiver coil 107 during the foreignobject detection interval. Furthermore, not only is the load of thepower receiver 105 reduced thereby making it easier to detect otherpower loss but often more importantly the power receiver 105 enters amore well-defined or certain state in which the impact of loadvariations on the electromagnetic test signal is reduced.

It will be appreciated that the loading of the receiver coil 107 may notbe completely switched off during the foreign object detection interval.For example, the power receiver 105 may still extract power for e.g.operating some internal circuitry. Thus, the load controller 309 may bearranged to disconnect a load from loading the receiver coil 107 whilestill allowing the receiver coil 107 to be loaded by one or more otherloads. Indeed, the loading of the receiver coil 107 can be considered asbeing comprised of a load which is disconnected by the load controller309 during the foreign object detection interval and a load which is notdisconnected by the load controller 309. Thus, the load 303 can beconsidered to represent the load that is disconnected by the receivercoil 107 during the foreign object detection interval. This load mayinclude both an external or internal load for which the power transferis established but may also include for example internal controlfunctionality temporarily switched off during the foreign objectdetection interval.

In some embodiments. the switchable load may e.g. be disconnected by areduction of the induced voltage at the input of the rectifier B1 whileat the same time maintaining a high voltage level at the output of therectifier by means of stored energy at the switchable load (which couldbe a battery), and/or at the capacitor C1. This may stop the currentthrough the rectifier B1 and therefore may effectively disconnect theswitchable load.

The power receiver 105 includes a power controller 311 which is arrangedto establish a power control loop with the power transmitter 101.Specifically, the power controller 311 can transmit power controlmessages to the power transmitter 101 and in response the powertransmitter 101 may change the power level of the power transfer signalduring the power transfer time intervals. Typically, the powercontroller 311 may generate power control error messages which indicatea request for the power transmitter 101 to increase or decrease thepower level. The power controller 311 may determine the appropriateerror messages by comparing a measured value to a reference value.During power transfer, the power controller 311 may compare the providedpower level with the required power level and request an increased ordecreased power level based on this comparison.

As previously mentioned, the system applies a repeating time frameduring the power transfer phase where the time frame comprises at leastone power transfer time interval and on foreign object detection timeinterval. An example of such a repeating time frame is illustrated inFIG. 5 where power transfer time intervals are indicated by PT andforeign object detection time intervals are indicated by D. In theexample, each time frame FRM comprises only one foreign object detectiontime interval and one power transfer time interval and these (as well asthe time frame itself) have the same duration in each frame. However, itwill be appreciated that in other embodiments, other time intervals mayalso be included in a time frame (such as e.g. communication intervals)or a plurality of foreign object detection time intervals and/or powertransfer time intervals may be included in each time frame. Furthermore,the duration of the different time intervals (and indeed the time frameitself) may in some embodiments vary dynamically.

In the approach, the foreign object detection and the power transfer isthus separated in the time domain thereby resulting in reducedcross-interference from the power transfer to the foreign objectdetection. Thus, the variability and uncertainty resulting fromvariations in the operating conditions for the power transfer can beisolated from the foreign object detection resulting in a more reliableand accurate foreign object detection.

In the power transfer signal time interval, the power transmitter isthus arranged to perform power transfer during the power transfer timeinterval of the time frames of the power transfer phase. Specifically,during these time intervals, the power transmitter 101 and the powerreceiver 105 may operate a power control loop (the power control loopmay be based on communication within the power transfer signal timeinterval or may e.g. be based on communication outside of the powertransfer signal time interval, such as in dedicated communication timeintervals. For example, each foreign object time interval may beseparated by a plurality of alternating power transfer signal timeintervals and communication time intervals). Thus, the level of thepower being transferred may be dynamically varied. In the foreign objectdetection time intervals of the time frames of the power transfer phase,at least one parameter of the drive signal, and thus of theelectromagnetic test signal, is set to a value determined during anadaptation operation performed prior to the power transfer phase. Thus,in the foreign object detection time interval, the parameter may be setto a predetermined value (i.e. being determined prior to the powertransfer phase). In contrast, the parameter may not be constrained tothis predetermined value during power transfer time intervals.

For example, during a power transfer time interval, the system mayoperate a power control loop which allows the power level of the powertransfer signal to be varied in response to power control messages fromthe power receiver. The power control loop may control/vary at least oneof a current, voltage, and frequency of the drive signal/power transfersignal. In contrast, during a foreign object detection time interval,the parameter varied by the power control loop during the power transfertime interval may be set to a predetermined value determined prior tothe power transfer phase.

In many embodiments where the same coil is used for both the powertransfer signal and the electromagnetic test signal, the powertransmitter may be arranged to reduce the level of the power transfersignal during the foreign object detection time interval relative to thepower transfer time interval. In many situations, the power level of thepower transfer signal may be allowed to increase to high levels, such ase.g. to levels of 10-100 W, or even substantially higher in manyapplications (e.g. for power transfer to kitchen appliances). However,during a foreign object detection time interval, the power level of thegenerated electromagnetic signal may be reduced to a predetermined levelthat is much lower than the current or maximum allowable power duringthe power transfer time interval. For example, the power level may beset to a predetermined level not exceeding 1 W. In other words, thepower of the electromagnetic test signal during the foreign objectdetection time interval may be constrained to a power level that issubstantially (e.g. by a factor of no less than 2, 5, or 10) lower thana maximum allowed power level of the power transfer signal during thepower transfer time interval.

Further, the power receiver 105 is arranged to reduce the load of thegenerated electromagnetic signal/field during the foreign objectdetection time interval relative to during the power transfer timeinterval, i.e. the power receiver 105 is arranged to decrease theloading of the power receiver 105 of the electromagnetic test signalduring the foreign object detection time interval relative to theloading of the power transfer signal during the power transfer timeinterval. Specifically, in the example of FIG. 3 the power receiver 105is arranged to disconnect the switchable load during the foreign objectdetection time interval and to connect it during the power transfer timeinterval. Thus, during the foreign object detection time interval, thepower receiver 105 may switch off (typically) the main load and indeedin many embodiments only a minimal load required for the continuedoperation of the power receiver 105 may be maintained.

In the example of FIG. 4, the switch S1 may be used to disconnect theload during the foreign object detection time interval. It will beappreciated that in embodiments where the switchable load 303 requires amore constant power provision, the switch S1 may be positioned beforethe capacitor C1 or another energy reservoir may be provided afterswitch S1 to supply the switchable load 303 with power during theforeign object detection time interval (or e.g. the previously describedapproach of reducing the induced voltage at the input of the rectifierB1 while at the same time maintaining a high voltage level at the outputof the rectifier B1 by means of stored energy at the switchable load(e.g. a battery), and/or at the capacitor C1 may be used).

The power receiver 105 may accordingly reduce a load of the powerreceiver during the foreign object detection time interval.Specifically, the load of the electromagnetic test signal by the powerreceiver during the foreign object detection time interval will be lessthan the load of the power transfer signal by the power receiver duringthe power transfer time interval (the load may e.g. be considered theeffective resistive impedance of respectively the transmitter coil 103and the test coil 209 during the power transfer time interval and theforeign object detection time interval respectively). Typically, thepower transfer signal and the electromagnetic test signal will havecorresponding properties and thus both induce a signal in the receivecoil 107. Therefore, disconnecting the switchable load 303 during theforeign object detection time interval will reduce the load of theelectromagnetic test signal relative to the load that is experienced bythe power transfer signal (and thus would be experienced by anelectromagnetic test signal) generated during the power transfer timeinterval when the load is connected.

The disconnection of the switchable load 303 not only reduces the loadof the electromagnetic test signal but may also provide for this load tobe more predictable and to have reduced variation. Typically, the loadof a power transmitter by a power receiver may vary substantially notonly from application to application, but also as a function of time forthe same application and power transfer session. The power control loopis operated during the power transfer phase to adapt to such variations.However, by introducing a foreign object detection time interval inwhich the load may be disconnected (or otherwise set to e.g. apredetermined level), it is possible to enter the power receiver into areference mode in which the loading of the electromagnetic field is morepredictable. Thus, the foreign object detection tests can be performedbased on the assumption that the power receiver is in this reference ortest mode, and thus e.g. a predetermined loading of the electromagnetictest signal can be assumed. The approach may thus not only allow for theloading by the power receiver 105 to be reduced (thereby improvingaccuracy by the relative impact of any foreign objects being higher) butalso allows this to be more predictable thereby facilitating thecompensation for the presence of the power receiver during the foreignobject detection test.

In addition to applying the time frame comprising specific foreignobject detection time intervals, the system also applies an approachwherein the value of one or more parameters (or properties) of thegenerated electromagnetic test signal is adapted based on a pre-powertransfer phase adaptation process. This adaptation process thusdetermines a preferred value for one or more of theparameters/properties of the electromagnetic test signal prior to thepower transfer phase and then applies this preferred value during theforeign object detection time intervals of the subsequent power transferphase. Further, the determination of the parameter is based oninformation transmitted from the power receiver 105 to the powertransmitter 101.

Thus, during an adaptation interval prior to the power transfer phase,the power transmitter 101 enters a foreign object detectioninitialization mode in which a preferred value for a parameter of theelectromagnetic test signal is determined based on one or more messagesfrom the power receiver 105.

Similarly, the power receiver controller 301 is arranged to control thepower receiver 101 to, during the adaptation interval prior to the powertransfer phase, operate in a foreign object detection initializationmode in which the power receiver 101 transmits at least one message tothe power transmitter 101.

This is illustrated in FIG. 6 which in addition to the power transferphase (PTP) also illustrates the adaptation time interval ADP in whichthe power transmitter 101 and power receiver 105 may enter a foreignobject detection initialization mode to determine a preferred value forone or more parameters of the electromagnetic test signal to be appliedduring one or more, and typically all, foreign object detection timeintervals of the subsequent power transfer phase.

The approach may further allow the foreign object detection tests of thesubsequent foreign object detection time intervals of the power transferphase to be performed under more predictable and controlled conditionswith reduced variability and uncertainty. For example, the parameter ofthe electromagnetic test signal may be set to a value that correspondsto a reference condition for which a property of the power receiver 101is known. E.g., the loading by the power receiver 105 on anelectromagnetic test signal resulting in a given induced signal level atthe power receiver 105 may be determined during design/manufacturing andstored in the power receiver 105. During use, the power receiver 105may, when operating in the foreign object detection initialization mode,transmit one or more messages to the power transmitter 101 whichprovides information on the setting of the drive signal to achieve thisinduced signal level as well as the corresponding loading by the powerreceiver 105. During the foreign object detection time intervals of thepower transfer phase, the power transmitter 101 may then set the drivesignal parameter (e.g. the signal level) to the appropriate value, andthe foreign object detector 207 may e.g. perform a power loss analysisforeign object detection test which includes a compensation for theknown/estimated power loss by the power receiver 105.

Thus, the system of FIGS. 1-4 provides for a much improved foreignobject detection test approach where the foreign object detection testsare performed under much more controlled conditions thereby allowingmore accurate and reliable foreign object detection tests to beperformed.

The parameter being set based on the foreign object detectioninitialization mode operation may depend on the preferences andrequirements of the individual embodiment and application scenario.Typically, the power transmitter 101 may be able to determine apreferred value for at least one of a voltage, current, and frequency ofthe test drive signal, and thus of the electromagnetic test signal.

For example, in some embodiments, the message received from the powerreceiver 105 may indicate a required magnetic field strength at a givendistance from the transmitter coil 103 (for example, the power receiver105 may indicate a required magnetic field strength at a distancecorresponding to the expected distance from the test coil 201 (which maybe assumed to be collocated with the transmitter coil 103) to thereceive coil 107 when the power receiver 105 is optimally positioned onthe power transmitter 101). The power transmitter 101 may convert thisrequired magnetic field strength into a required test drive signalcurrent that will give rise to a field strength corresponding to thatrequired. The power receiver 105 may further provide an indication ofthe power loss in the power receiver 105 for this field strength(specifically the power loss from friendly metal and internal circuitryand with the switchable load 303 disconnected).

The power transmitter 101 may then proceed to set the current of thetest drive signal to this value during the foreign object detection timeintervals of the subsequent power transfer phase and when performingpower loss based foreign object detection tests, it may determine thepower loss as the power of the test drive signal minus the power lossexpected from the power receiver 101.

In many embodiments, the message received from the power receiver 105may comprise an indication of a property of the power receiver 105, andthe adapter 213 may be arranged to determine the preferred value for thegiven parameter of the test drive signal/electromagnetic test signal inresponse to the indication of the property of the power receiver 105.

For example, as indicated above, the message may indicate a power lossin friendly metal and the loading by the power receiver circuitry for agiven reference operating condition. As another example, the messageindication may simply indicate a type or class of power receiver and theadapter 213 may be arranged to e.g. retrieve corresponding predeterminedparameter values for the electromagnetic test signal from a local storethat comprises suitable values for a range of types/classes of powerreceivers.

In some embodiments, the indication may be an indication of e.g. aresonance frequency (or frequency range) for the power receiver 105.This may for example be used by the power transmitter 101 to set thefrequency of the test drive signal/electromagnetic test signal to theindicated frequency, and indeed may in some embodiments allow the powertransmitter 101 to adjust a resonance frequency of an output resonancecircuit involving the test coil 209. Such a scenario may be particularlysuitable for foreign object detection tests that are based on measuringthe Q factor (or other quality measure) of the output resonance circuit.

In some embodiments, the message received from the power receiver 105may comprise an indication of an expected impact of the power receiveron a reference test drive signal, and the adapter 213 may be arranged todetermine the preferred value and/or to adapt the foreign objectdetection test in response to the indication of the expected impact ofthe power receiver.

For example, as previously described, the power receiver 105 mayindicate a preferred setting for e.g. the strength of theelectromagnetic test signal and consequently a preferred setting for thecurrent through the test coil 209. The power transmitter 101 may thenprovide a reference test drive signal corresponding to this referenceelectromagnetic test signal.

Alternatively or additionally, the power receiver 105 may indicate e.g.the power loss in the power receiver 105 during a foreign objectdetection time interval (i.e. with the switchable load 303 beingdisconnected) when a reference electromagnetic test signal is provided.It may then as previously described adapt the foreign object detectiontest, e.g. by subtracting the reported power loss in the power receiver105 from the measured power level for the test drive signal.

As another example, the power receiver 105 may provide an indication ofa quality of a power receiver resonance circuit comprising the receivecoil 107. For example, an indication of a resistive load or Q factor maybe provided. The adapter 213 may then adapt e.g. a foreign objectdetection test based on measuring the Q factor of the power transmitteroutput resonance circuit comprising the test coil 209 based on thereported power receiver Q factor. For example, a lower reported Q factorcan reduce the threshold for detecting whether a reduced quality measurefor the output resonance circuit may be indicative of a foreign objectbeing present.

Thus, in some embodiments, the power receiver 105 may transmit data thatmay be indicative of the impact of the power receiver 105 on theelectromagnetic test signal when the power transmitter 101 provides anexpected reference electromagnetic test signal, i.e. when theelectromagnetic test signal has the expected reference properties.

The power transmitter 101 may use this to determine the expected valuesfor parameters of the test drive signal/electromagnetic test signal, andthus may adapt the foreign object detection test, and specifically thedecision criteria for foreign object detection, accordingly.

The information provided by the power receiver 105 may in manyembodiments provide or allow a determination of one or more of thefollowing:

-   -   an expected power dissipation by power receiver (typically        including friendly metal),    -   an expected (minimum) Q factor, and/or    -   an expected maximum resonance frequency.

The adapter 213 may then adapt the test drive signal and/or the foreignobject detection test in response.

In some embodiments, the message from the power receiver may comprise anindication of a difference between a current power receiver operatingvalue and a test reference power receiver operating value. For example,the message may comprise an indication of a difference between a currentlevel of the signal induced in the receive coil 107 and thereference/desired level of the signal induced in the receive coil 107.The power transmitter 101 may be arranged to modify a parameter of thepower transfer signal in response to the indication, and specificallymay be able to drive the value towards a level where the message fromthe power receiver indicates that the current operating value is equalto the desired operating value.

As an example, during the pre-power transfer phase adaptation phasewhere the power transmitter 101 and the power receiver 105 are bothoperating in the foreign object detection initialization mode, the powerreceiver 105 may measure e.g. the current amplitude of the voltage overthe rectifier bridge B1. It may compare this to a desired level and senda message to the power transmitter 101 which indicates the difference.For example, if the measured voltage is only half the desired level, itmay transmit a request for the signal level of the electromagnetic testsignal to be increased by 6 dB. The power transmitter 101 may inresponse to receiving the message set the preferred value for the levelof the test drive signal to be 6 dB higher than the current value. Thispreferred value may then be used for generating the electromagnetic testsignal during the foreign object detection time intervals of the powertransfer phase.

As another example, the induced voltage at the power receiver may bemeasured using a dedicated coil. This may provide a direct indication ofthe field to which the friendly metal is exposed. The measurement resultmay be transmitted to the power transmitter 101 which may then adapt thetest drive signal in response.

In some embodiments, the process of the power receiver 105 transmittingmessages indicating the difference between a current operating value anda reference value may be iterated and specifically the power receiver105 and power transmitter 101 may during the adaptation intervalimplement a control loop that drives the test drive signal towards thedesired level for the power receiver 105 to be operating at the desiredreference operating point, e.g. typically at the desired level of signalinduced in the receive coil 107. The power receiver 105 may simplyrepeatedly transmit indications for an increase or decrease in the levelof the electromagnetic test signal. The resulting value for the powertransfer signal may then be stored as the preferred value and this maybe applied during the foreign object time intervals of the powertransfer phase.

In more detail, during the adaptation interval, the power receivercontroller 301 may arranged to determine a difference between a level ofa signal induced in the power receiver coil and a reference level. Thelevel may typically be determined as a voltage level (specifically alevel of an induced voltage) but could in other embodiments e.g., be apower level (specifically a level of an induced signal power) or acurrent level (specifically a level of an induced current). It will beappreciated that any suitable indication of a level of an induced signalmay be used.

In many embodiments, the power receiver controller 301 is arranged tocompare a voltage level indication of an induced signal to a referencevoltage, and to generate test signal control messages based on thiscomparison. If the voltage is below the reference value, a test signalcontrol message requesting the level of the electromagnetic test signalto be increased is transmitted, and if it is above the reference value,a test signal control message requesting the level of theelectromagnetic test signal to be decreased is transmitted. In response,the adapter 213 increases or decreases the test drive signal level toprovide a corresponding change in the electromagnetic test signal.Specifically, rather than merely transmitting a single message, thepower receiver 105 and the power transmitter 101 may effectivelyimplement a control loop during the adaptation interval which drives thetest drive signal to generate the desired reference operation conditionfor the foreign object test. The preferred value of the parameter of thetest drive signal may be set to the end value after the loop hasconverged to a given value corresponding to the reference condition.

In this way, the power receiver 105 can control the level of theelectromagnetic test signal such that the level of the induced signal isdriven towards the reference value. Specifically, the voltage over thereceiver coil 107 may be driven to be equal to a given referencevoltage.

The approach thus allows for the power receiver 105 to be in control ofestablishing a predetermined configuration in which a typicallypredetermined load is provided and the induced signal, and specificallythe induced voltage, is at a predetermined level. Thus, a referenceoperation condition is set up for the power receiver 105 (by the powerreceiver 105 itself).

In some such systems, the message(s) transmitted to the powertransmitter 101 may comprise an indication of a loading of the powertransmitter 101 by the power receiver 105 when the power receiver 105 isoperating at the given reference operating point for the foreign objectdetection, i.e. when the switchable load 303 is disconnected and theinduced signal level is at/equal to the reference level. Specifically,the indication may be indicative of the power that would be extractedfrom the electromagnetic test signal when the system is operating in ascenario and operating configuration with the switchable load 303 beingdisconnected and the induced signal in the power receiver coil being atthe reference level.

This loading indication accordingly provides information on the effectthat the power receiver 105 has on the electromagnetic test signalduring the foreign object detection time interval. During this interval,the power transmitter 101 sets the value of the test drive signal suchthat the resulting level of the induced signal is substantially at thereference value (when no foreign object is present and with theswitchable load 303 being disconnected).

The loading indication may typically be a predetermined loadingindication. It may be based on assumptions that the induced signal levelis at the reference level and that the switchable load 303 isdisconnected. In many embodiments, the predetermined loading indicationmay indeed be a stored value which is transmitted to the powertransmitter 101 simply by being retrieved from memory and transmittedwithout being modified by any measurement or modification based oncurrent conditions. Indeed, in many embodiments, the only measurementmade is that of the induced signal level such that this can be driventowards the reference level. However, in many embodiments, thepredetermined loading indication is also independent of this, i.e. thepredetermined loading indication is retrieved and transmitted to thepower transmitter 101, and the measurements of the induced signal arethen used to drive the level to the reference level such that the actualoperating condition is equal to that assumed for the predeterminedloading indication.

For example, during the design or manufacturing phase for a powerreceiver, this may be positioned in a test set-up wherein anelectromagnetic signal is provided and wherein it is assured that noother objects are present to extract power from the electromagneticdetection signal. The power receiver may be set to a configurationcorresponding to the switchable load 303 being disconnected (for exampleno load may be included or a switch of the power receiver may disconnectthe load). The power receiver may then be operated in a foreign objectdetection mode with the test setup generating an electromagnetic testsignal at the appropriate reference levels. When a sufficiently stableoperation is achieved, the power extracted from the electromagnetic testsignal is measured (e.g. by measuring the power of a drive signaldriving a coil generating the electromagnetic signal). The measurementcan be performed under closely controlled conditions, and with highlyaccurate measurement devices and thus the extracted power can be veryaccurately measured. The measured value may then be programmed into themanufactured power receivers and used as the predetermined loadingindication.

The predetermined loading indication may thus be a predetermined valuewhich is transmitted to the power transmitter and which provides anindication of the loading that the power receiver is expected toexercise on the electromagnetic test signal when the power receiver 105is operating in the foreign object detection operating configuration.The value is not merely a measurement of the actual power of the signalinduced in the receiver coil 107 but is a predetermined value that mayinclude e.g. loading caused by conductive elements of the power receiver105 itself (often referred to as friendly metal). Thus, the messagetransmitter 313 in such embodiments transmits a predetermined loadingindication that indicates the expected loading of the electromagneticdetection signal by the presence of the power receiver 105 operating inthe foreign object detection configuration.

The message receiver 205 of the power transmitter 101 may receive thepredetermined loading indication and forward this to the foreign objectdetector 207. The foreign object detector 207 may perform a foreignobject detection test by comparing the power level of the generatedelectromagnetic test signal, i.e. the transmit power level, to thepredetermined loading indication. In many embodiments, the foreignobject detector 207 may simply subtract the predetermined loadingindication from the transmitter power level. If the result exceeds agiven threshold, the foreign object detector 207 may determine that aforeign object has been detected, and otherwise it is considered that noforeign object has been detected.

Specifically, the power transmitter may determine its transmitted powerlevel during the foreign object detection timing interval for which areceived power is reported from the power receiver 105 by thepredetermined loading indication. Based on these values, the foreignobject detector 207 can calculate the difference between the transmittedand received power and check if the difference is within a smalltolerance range. If the difference is outside the range, the foreignobject detector 207 indicates that a foreign object has been detected.If it is within the range, the foreign object detector 207 indicatesthat no detection of a foreign object has occurred. This range may bechosen such that the power dissipation in a metal object not detected bythis power difference is considered acceptably low.

Of course, it will be appreciated that other, and typically morecomplex, decision criteria may be used in other embodiments.

As another example, the power transmitter 101 and power receiver 105 mayperform an operation which adapts the frequency of the test drive signalin response to the messages from the power receiver 105. For example,the power transmitter 101 could sequentially set the frequency to arange of values and the power receiver 105 could transmit an indicationof which setting resulted in the highest received value (correspondingto the most energy efficient transfer and optimally to the drive signalfrequency and the frequencies of the resonance circuits of the powertransmitter 101 and the power receiver 105 being equal). This frequencymay then be used for the subsequent foreign object detection testsduring the power transfer phase,

In some embodiments, the adapter 213 may further be arranged todetermine the preferred value in response to constraint of the foreignobject detection test of the foreign object detector. In manyembodiments, the constraint may be at least one of a constraint on afrequency of the test drive signal and a constraint of a minimum signallevel for the test drive signal.

For example, in many embodiments, it is desirable for the test drivesignal and the electromagnetic test signal to be as weak as possible asit will reduce any heat induced in potential foreign objects, willreduce power consumption, reduces the design requirements for e.g. thetest coil 209 etc. However, at the same time the foreign objectdetection tends to be more accurate the higher the signal level/fieldstrength is. Accordingly, in some embodiments, the adapter 213 maydetermine the preferred value for the signal level of the test drivesignal but with that level being subject to a constraint of a minimumsignal level. Thus, regardless of the information received from thepower receiver 105, the adapter 213 will not set the preferred signallevel that is used for the test drive signal/electromagnetic test signalduring the foreign object detection time intervals of the power transferphase to be below this given threshold.

As another example, in embodiments wherein the test coil 209 is part ofan output resonance circuit, the adapter 213 may be arranged to set thefrequency of the test drive signal as requested by the power receiverbut subject to the constraint that this frequency is within a givenfrequency range. This may specifically ensure that the test outputresonance circuit comprising the test coil 209 is not driven too farfrom its optimal resonance frequency.

In some embodiments, the adapter 213 may further be arranged to preventthe power transmitter from entering the power transfer phase if thepreferred value does not meet a criterion. For example, a preferredvalue may be determined based on the information received from the powerreceiver 105 and this process may lead to a value that is outside theexpected range for the parameter. For example, a power receiver 105 maycontinuously request increased power of the electromagnetic test signaluntil this is at a very high level. This may be indicative of ananomalous situation occurring, e.g. caused by an error in the powertransmitter 101, the power receiver 105, or the presence of a foreignobject. Accordingly, the adapter 213 may consider the determinedparameter to be outside an operating range and the power transmitter 101may be prevented from entering the power transfer phase therebypreventing that high level power transfer is begun in an errorsituation. Thus, a more resilient and reliable power transferinitialization can be achieved.

In some embodiments, the foreign object detector 207 may be arranged toadapt a parameter of the foreign object detection test in response to ameasured value of the drive signal when in the foreign object detectioninitialization mode.

For example, when the preferred value has been determined, a test drivesignal may be generated with the parameter set to the preferred value.Specifically, a test drive signal may be generated with the level andfrequency that will be used during the foreign object detection timeintervals of the power transfer phase. A property of the drive testsignal may be measured for this setting, such as e.g. a voltage orcurrent that provides an indication of the total power extracted fromthe electromagnetic test signal. The measured value may be stored andused as a reference for subsequent measurements during the powertransfer phase. Thus, the foreign object detection may be based on acomparison to a reference measurement made during the operation in theforeign object detection initialization mode, i.e. during the pre-powertransfer phase adaptation interval. As a specific example, the foreignobject detection test may consider a foreign object to be detected ifthe measured extracted power during a foreign object detection timeinterval of the power transfer phase exceeds the measured extractedpower during the adaptation interval by a given threshold.

Such an approach may typically provide an improved adaptation to thelocal conditions and may establish a typically accurate basis fordetecting changes occurring by a foreign object being brought into thepresence of a power transmitter during a power transfer phase. It mayallow a more accurate foreign object detection in many scenarios.

In many embodiments, the power transmitter 101 may further be arrangedto perform foreign object detection tests prior to initializing thepower transfer phase. For example, as illustrated in FIG. 6, thepre-power transfer phase may include one or more foreign objectdetection time intervals following the adaptation interval and prior toentering the power transfer phase.

The foreign object detection tests performed during these pre-powertransfer phase foreign object detection time intervals may be performedusing the parameter values for the test drive signal determined when inthe foreign object detection initialization mode operation, i.e. in theadaptation interval. As such, the foreign object detection tests priorto entering the power transfer phase may be the same as those performedduring the power transfer phase, i.e. the same parameters may be usedboth for the foreign object detection test and for the generated testdrive signal/electromagnetic test signal.

In such approaches the foreign object detector 207 may thus also performforeign object detection tests in one or more initial test intervalsprior to the power transfer. Furthermore, the power transmitter may bearranged to enter the power transfer phase only if the foreign objectdetection tests performed in these initial test intervals indicate thatno foreign object detection is present. Such an approach may reduce therisk of high level power transfer being started in scenarios whereinthis may cause excessive heat in a foreign object. Furthermore, thetests can be performed with all the benefits of the foreign objectdetection tests performed during the power transfer phase (specificallywith the system operating in a dedicated reference configuration).

It will be appreciated that the reaction of the power transmitter 101 toa foreign object detection test being positive, i.e. indicating that aforeign object is present, may be different in different embodiments.Indeed, in many embodiments, the power transmitter 101 may be arrangedto terminate the power transfer if the foreign object detection testindicates that a foreign object may be present.

In some embodiments, the foreign object detection is arranged tore-enter the power transmitter into the foreign object detectioninitialization mode if the foreign object detection test indicates thata foreign object may be present. The power transmitter 101 may furthertransmit an indication to the power receiver 105 that it has enteredthis foreign object detection initialization mode, and in response thepower receiver 105 may enter the foreign object detection initializationmode. Thus, in some embodiments, the system may effectively return tothe adaptation interval/mode of operation in response to a positiveforeign object detection test. This may result in the system repeatingthe process of entering a predetermined configuration and determining apreferred value for the parameter(s) of the test drive signal (and thusthe electromagnetic test signal). Following this process, the powertransmitter 101 may again perform a number of foreign object detectiontests and if these indicate that no foreign object is present, it mayproceed to re-enter the power transfer phase.

The approach may provide improved resilience and more robust operationin many embodiments. It may for example allow the system toautomatically recover from situations where e.g. the positive foreignobject detection test is not due to the presence of a foreign object butrather occurs due to an inaccurate foreign object detection test beingbased on inaccurate assumptions. For example, if the foreign objectdetection test occurs due to the generated electromagnetic test signalbeing too high and therefore the actual power loss in friendly metal ofthe power receiver being higher than expected, the approach may allow anautomatic “re-calibration” of the test drive signal and the foreignobject detection test. If this is successful (with no foreign objectdetection subsequently being detected), the system may restart the powertransfer.

Such event can for example occur when the position of the power receiverrelative to the test coil of the power transmitter has been changed.Such position change can cause the friendly metal of the power receiverto change the measured signal of the power transmitter, and thus affectthe foreign object test. The described approach may cause the system toautomatically re-calibrate to the changed conditions.

In some embodiments, the power transmitter 101 may be arranged to set aparameter of the power transfer/the power transfer signal in response toa measurement of the test drive signal during the foreign objectdetection time interval. Specifically, the adapter 213 may be arrangedto set a maximum level for the power transfer signal during the powertransfer interval in response to a measurement of the test drive signalduring the foreign object detection interval.

For example, the power of the test drive signal may be determined basedon measurements of the current and/or voltage of the test drive signal.This power level reflects the power that is being extracted from theelectromagnetic test signal. This may be compared to the expected powerto be extracted from the electromagnetic test signal by the powerreceiver 105. If the two values are very close, it can be assumed thatthere is only the power receiver 105 present and a high power level ofthe power transfer signal during the power transfer time interval willbe allowed. However, if the difference is higher, but still not highenough to result in a positive foreign object detection, the measurementmay still reflect a risk that some power is lost in another entity thanthe power receiver 105. The power transmitter 101 may in this casereduce the maximum power level of the power transfer signal such thatthe potential power loss outside the power receiver is ensured to besufficiently low to not e.g. risk heating to excessive temperatures.

Thus, in some embodiments, the power transmitter 101 may use themeasurements during the foreign object detection time interval todetermine the maximum amplitude (and/or frequency) of the power transfersignal considered acceptable in that it is assumed that any resultingpower loss outside the power receiver 105 will still result in potentialtemperature rises that are within acceptable limits. The powertransmitter 101 may then limit the power transfer signal to this maximumvalue and e.g. generate an alarm or transmit a warning message to thepower receiver 105 if this tries to increase the power level above thisdetermined maximum level.

It will be appreciated that the above description for clarity hasdescribed embodiments of the invention with reference to differentfunctional circuits, units and processors. However, it will be apparentthat any suitable distribution of functionality between differentfunctional circuits, units or processors may be used without detractingfrom the invention. For example, functionality illustrated to beperformed by separate processors or controllers may be performed by thesame processor or controllers. Hence, references to specific functionalunits or circuits are only to be seen as references to suitable meansfor providing the described functionality rather than indicative of astrict logical or physical structure or organization.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. The inventionmay optionally be implemented at least partly as computer softwarerunning on one or more data processors and/or digital signal processors.The elements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units, circuits andprocessors.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term comprising does not exclude the presence ofother elements or steps.

It will be appreciated that the reference to a preferred value does notimply any limitation beyond it being the value determined in the foreignobject detection initialization mode, i.e. it is preferred by virtue ofit being determined in the adaptation process. The references to apreferred value could be substituted for references to e.g. a firstvalue.

Furthermore, although individually listed, a plurality of means,elements, circuits or method steps may be implemented by e.g. a singlecircuit, unit or processor. Additionally, although individual featuresmay be included in different claims, these may possibly beadvantageously combined, and the inclusion in different claims does notimply that a combination of features is not feasible and/oradvantageous. Also the inclusion of a feature in one category of claimsdoes not imply a limitation to this category but rather indicates thatthe feature is equally applicable to other claim categories asappropriate. Furthermore, the order of features in the claims do notimply any specific order in which the features must be worked and inparticular the order of individual steps in a method claim does notimply that the steps must be performed in this order. Rather, the stepsmay be performed in any suitable order. In addition, singular referencesdo not exclude a plurality. Thus references to “a”, “an”, “first”,“second” etc. do not preclude a plurality. Reference signs in the claimsare provided merely as a clarifying example shall not be construed aslimiting the scope of the claims in any way.

1. A power transmitter for wirelessly providing power to a powerreceiver comprising: a transmitter coil, wherein the transmitter coilgenerates a power transfer signal; a driver, wherein the drivergenerates a drive signal for the transmitter coil, wherein the driver isarranged to generate the drive signal during a power transfer phase,wherein the drive signal uses a repeating time frame, wherein therepeating time frame comprises at least a power transfer time intervaland a foreign object detection time interval; a receiver, wherein thereceiver is arranged to receive messages from the power receiver; a testcoil, wherein the test coil is arranged to generate an electromagnetictest signal; a test generator, wherein the test generator is arranged togenerate a test drive signal for the test coil such that theelectromagnetic test signal is provided during the foreign objectdetection time interval; a foreign object detector, wherein the foreignobject detector is arranged to perform a foreign object detection testin response to a measured parameter for the test drive signal; anadapter, wherein the adapter is arranged to control the powertransmitter to operate in a foreign object detection initialization modeprior to entering the power transfer phase, wherein a preferred value ofa signal parameter for the test drive signal is determined in responseto at least a first message received from the power receiver, during theforeign object detection initialization mode wherein the test generatoris arranged to set the signal parameter of the test drive signal to thepreferred value during the foreign object detection time interval. 2.The power transmitter of claim 1, wherein the first message comprises anindication of a property of the power receiver.
 3. The power transmitterof claim 1, wherein the first message comprises an indication of anexpected impact of the power receiver on a reference electromagnetictest signal.
 4. The power transmitter of claim 1, wherein the firstmessage comprises an indication of a constraint for the signal parameterfor the test drive signal.
 5. The power transmitter of claim 1, whereinthe first message comprises an indication of a difference between acurrent power receiver operating value and a test reference powerreceiver operating value.
 6. The power transmitter of claim 1, whereinthe adapter is arranged to determine the preferred value in response toa constraint of the foreign object detection test of the foreign objectdetector.
 7. The power transmitter of claim 6, wherein the constraint isat least one of a minimum signal level and a constraint on a frequencyof the test drive signal.
 8. The power transmitter of claim 1, whereinthe test generator is arranged to generate the test drive signal withthe drive parameter of the test drive signal adapted to the preferredvalue in an initial test interval prior to the power transfer phase,wherein the foreign object detector is arranged to perform the foreignobject detection test in the initial test interval.
 9. The powertransmitter of claim 1, wherein if the foreign object detection test inthe foreign object detection time interval is indicative of a foreignobject being present, the foreign object detector is arranged tore-enter the power transmitter into the foreign object detectioninitialization mode.
 10. The power transmitter of claim 1, wherein theadapter is arranged to prevent the power transmitter from entering thepower transfer phase if the preferred value does not meet a criterion.11. The power transmitter of claim 1, wherein the foreign objectdetector is arranged to adapt a parameter of the foreign objectdetection test in response to a measured value of the drive signal whenin the foreign object detection initialization mode.
 12. The powertransmitter of claim 1, wherein the adapter is arranged to set a maximumlevel for the power transfer signal during the power transfer intervalin response to a measurement of the test drive signal during the foreignobject detection interval.
 13. A wireless power transfer systemcomprising: a power transmitter, wherein the power transmitter isarranged to provide power to a power receiver via a power transfersignal, the power transmitter comprising: a transmitter coil, whereinthe transmitter coil generates the power transfer signal; a driver,wherein the driver generates a drive signal for the transmitter coil,wherein the driver is being arranged to generate the drive signal duringa power transfer phase, wherein the drive signal uses a repeating timeframe, wherein the repeating time frame comprises at least a powertransfer time interval and a foreign object detection time interval,wherein a power of the power transfer signal is reduced relative to thepower transfer time interval during the foreign object detection timeinterval; a receiver, wherein the receiver is arranged to receivemessages from the power receiver; a test coil, wherein the test coil isarranged to generate an electromagnetic test signal; a test generator,wherein the test generator is arranged to generate a test drive signalfor the test coil such that an such that the electromagnetic test signalduring the foreign object detection time interval; a foreign objectdetector, wherein the foreign object detector is arranged to perform aforeign object detection test in response to a measured parameter forthe test drive signal; an adapter, wherein the adapter is arranged tocontrol the power transmitter to operate in a foreign object detectioninitialization mode prior to entering the power transfer phase, whereina preferred value of a signal parameter for the test drive signal isdetermined in response to at least a first message received from thepower receiver, during the foreign object detection initialization modewherein the test generator is arranged to set the signal parameter ofthe test drive signal to the preferred value during the foreign objectdetection time interval; and the power receiver, the power receivercomprising: a power receiving coil, wherein the power receiving coil isarrange to extract power from the power transfer signal; a foreignobject detection controller, wherein the foreign object detectioncontroller is arranged to reduce a load of the power receiver during theforeign object detection time interval; a message transmitter, whereinthe message transmitter is arranged to transmit the first message to thepower transmitter.
 14. The wireless power transfer system of claim 13,wherein the power receiver further comprises a power receivercontroller, wherein the power receiver controller is arranged to controlthe power receiver to operate in a foreign object detectioninitialization mode in which the power receiver transmits at least onemessage to the power transmitter to bias the test drive signal towardscausing a reference condition at the power receiver.
 15. A method for apower transmitter wirelessly providing power to a power receiver,wherein the power transmitter comprises: a transmitter coil, wherein thetransmitter coil generates a power transfer signal, a test coil, whereinthe test coil is arranged to generate an electromagnetic test signal;and a receiver, wherein the receiver is arranged to receive from thepower receiver, the method comprising: generating a drive signal for thetransmitter coil, wherein the drive signal uses a repeating time frameduring a power transfer phase, wherein the repeating time framecomprises at least a power transfer time interval and a foreign objectdetection time interval; generating a test drive signal for the testcoil so as to provide the electromagnetic test signal during the foreignobject detection time interval; performing a foreign object detectiontest in response to a measured parameter for the test drive signal; andcontrolling the power transmitter to operate in a foreign objectdetection initialization mode in which a preferred value of a signalparameter for the test drive signal is determined in response to atleast a first message received from the power receiver prior to enteringthe power transfer phase, wherein the test drive signal is generatedwith the signal parameter set to the preferred value during the foreignobject detection time interval.
 16. The method as claimed in claim 15,wherein the first message comprises an indication of a property of thepower receiver.
 17. The method as claimed in claim 15, wherein the firstmessage comprises an indication of an expected impact of the powerreceiver on a reference electromagnetic test signal.
 18. The method asclaimed in claim 15, wherein the first message comprises an indicationof a constraint for the signal parameter for the test drive signal. 19.The method as claimed in claim 15, wherein the first message comprisesan indication of a difference between a current power receiver operatingvalue and a test reference power receiver operating value.
 20. Themethod as claimed in claim 15, further comprising: generating the testdrive signal with the drive parameter of the test drive signal adaptedto the preferred value in an initial test interval prior to the powertransfer phase, performing the foreign object detection test in theinitial test interval.